In recent year, radio communication systems typified by a mobile telephone system and so forth have offered diversified services, and therefore are required to transmit not only speech data but also large volume of data such as still image data and moving image data. To realize transmission of large volume of data, studies are actively underway related to a pre-coding technique and an adaptive modulation and coding (AMC) technique directed to improving the efficiency of use of frequencies and reducing interference.
With the pre-coding technique, a radio communication apparatus in the data transmitting side performs linear signal processing or non-linear signal processing on transmission data, based on channel information (CSI: channel state information) estimated in a radio communication apparatus in the data receiving side. By this means, a radio communication apparatus in the data transmitting side is able to transmit transmission data with interference components suppressed in advance, which is added in a channel (propagation path). That is, it is possible to provide a channel equivalent to an optimal channel by using the pre-coding technique. The pre-coding technique is being studied also by 3GPP LTE (3rd Generation. Partnership Project Long-Term Evolution).
On the other hand, with the adaptive modulation and coding technique, for example, in order to satisfy a required QoS (quality of service), the radio communication apparatus in the data transmitting side adaptively controls the modulation scheme and the modulation and coding scheme (MCS) of transmission data based on CSI estimated in the radio communication apparatus in the data receiving side.
As described above, to use the pre-coding technique and the adaptive modulation and coding technique, the radio communication apparatus in the data transmitting side needs to have CSI estimated in the radio communication apparatus in the data receiving side. In a case of a frequency division duplex (FDD) system using different frequencies between the uplink and the downlink, in order to perform pre-coding or adaptive modulation and coding in the radio communication apparatus in the data transmitting side, the radio communication apparatus in the data receiving side needs to feed the estimated CSI of the downlink back to the radio communication apparatus in the data receiving side via the feedback channel. Therefore, an FDD system needs to prevent overload of the feedback channel due to an increase in the amount of feedback.
As a conventional technique for reducing the amount of feedback in the feedback channel, there is the technique for feeding hack CSI indicating path gains (power of paths) of only primary paths among a plurality paths having different delay times (for example, see Non-Patent Literature 1.) To be more specific, with this conventional technique, the radio communication apparatus in the data receiving side feeds, back to the radio communication apparatus in the data receiving side, CSI indicating path gains in the range including the primary range (for example, the range for the guard interval (GI) length) from the beginning path at the minimum delay time, among a plurality of path gains at each delay time. For example, this conventional technique defines proportion R of the path gain for the primary paths to the path gain for all paths (total path gain) (=the power with the primary path gain/the power with all path gains) [%]. Then, the radio communication apparatus in the data receiving side feeds, back to the radio communication apparatus in the data receiving side, CSI indicating path gains in the range including all path gains of R %, from the beginning path at the minimum delay time. That is, the remaining (100−R) % of all path gains is not reported to the radio communication apparatus in the data transmitting side. As a result of this, the amount of feedback is reduced in the feedback channel.